1. Field of the Invention
The invention relates to a method for separating and estimating multiple motion parameters in an X-ray angiogram image.
2. Description of the Related Art
Respiratory motion includes inspiration motion and expiration motion formed by rhythmic inflation and deflation of a thorax. The respiratory motion may cause translational motion of the heart in a three-dimensional space. In an X-ray angiography system, two-dimensional motion of coronary vessels may occur on angiography plane. Moreover, an angiography sequence may cover the entire coronary vessels during angiography, causing two-dimensional translational motion between different frames of the imaging sequence.
Therefore, a coronary angiogram image operates to record a projection of cardiac motion on a two-dimensional plane, and superposition of two-dimensional translational motion of coronary on the angiography plane caused by respiratory motion of a human body, and two-dimensional translational motion of the patient. To obtain an accurate two-dimensional angiogram, it is beneficial to subtract cardiac motion, the respiratory motion and translational motion of the patient.
A conventional method for separating the translational motion is to conduct image registration between frames. Medical image registration methods reported in some literatures normally comprise an external registration method and an internal registration method. The external registration method sets some obvious reference points prior to angiography, and traces them during the angiography. However, this kind of labeling method is normally invasive. The internal registration method is divided into a labeling method, a segmentation method and so on after angiography is completed. The labeling method selects some anatomical structure points from an angiogram for registration, but not all angiograms have such anatomical structure points. The segmentation method can enable registration via anatomical structure lines that are segmented, and can be applicable for both rigid models and deformation models. However, it can only extract motion of the anatomical structure line, and cannot extract rigid translational motion alone. However, motion of vessel extracted from the angiogram images contains translational motion of a patient, along with physiological motion and respiratory motion of the vessel itself. Even if some angiogram images contain skeletons with no physiological motion (such as spines) as internal labeling points and it is possible to enable registration via the internal labeling method, not all the angiogram images have such a feature for registration. As no such feature exists in an angiogram image, it is difficult to extract the translational motion. Moreover, extraction of the translational motion is seldom reported in the medical image field.
It has been suggested to set labeling points before extracting human's respiratory motion, and tracing those points in sequence. One feature of the respiratory motion is that as one person breaths, some organs in his body, such as a heart, a diaphragm and so on may move along with a lung in a three-dimensional space. Therefore, a motion curve obtained by tracing structure feature points that are not to move along with the heart can be equivalent to respiratory motion. Two common-used methods in X-ray angiography comprise manually tracing non-cardiac structure feature points in the angiogram image, and simultaneously recording motion of these points during angiography. Obviously, both these two methods have defects. The former one features poor applicability: since it is impossible to ensure that a labeling point meeting the above-mentioned requirement exists in each frame of the angiogram image, and to find the labeling point requires good understanding of human's anatomical structure, it is hard to extract the respiratory motion as there is no feature point in the angiogram image. Implementation of the latter one requires large amount of experiment for controlling, which is inappropriate for normal clinical application. In addition, there is still another method that is implemented under a dual-arm X-ray angiogram, and a principle thereof for separating the cardiac motion from the respiratory motion is to conduct three-dimensional reconstruction on coronary vessels of two angiogram images at different projection angles at the same time thereby obtaining three-dimensional distribution of the vessel at that time. After all angiogram images in one respiratory cycle are matched and reconstructed, a group of three-dimensional structure sequences can be obtained, and spatial displacement vectors therebetween constitute the respiratory motion. This method can obtain comparatively reliable respiratory motion, but cannot be widely applied to medical practice due to limitation of conditions of the dual-arm X-ray angiogram.
A typical method for extracting respiratory motion parameters from x-ray angiogram image discloses Fourier frequency-domain separation. However, a residual motion curve can be regarded as respiratory motion only after the cardiac motion is extracted. In addition, it is possible to extract translational motion and high-frequency motion, and as a result, respiratory motion is inaccurate, and robustness of the method is insufficient.